Function Overloading Feature

Warning

This feature has been
DEPRECATED as of PHP 7.2.0. Relying on this feature
is highly discouraged.

You might often find it difficult to get an existing PHP application
to work in a given multibyte environment. This happens because most
PHP applications out there are written with the standard string
functions such as substr(), which are known to
not properly handle multibyte-encoded strings.

mbstring supports a 'function overloading' feature which enables
you to add multibyte awareness to such an application without
code modification by overloading multibyte counterparts on
the standard string functions. For example,
mb_substr() is called instead of
substr() if function overloading is enabled.
This feature makes it easy to port applications that only support
single-byte encodings to a multibyte environment in many cases.

To use function overloading, set
mbstring.func_overload in php.ini to a
positive value that represents a combination of bitmasks specifying
the categories of functions to be overloaded. It should be set
to 1 to overload the mail() function. 2 for string
functions, 4 for regular expression functions. For example,
if it is set to 7, mail, strings and regular expression functions will
be overloaded. The list of overloaded functions are shown below.

It is not recommended to use the function overloading option in
the per-directory context, because it's not confirmed yet to be
stable enough in a production environment and may lead to undefined
behaviour.

I took over a project with this "feature" enabled... took me a while to figure out why strings behaved non language standard. So, if you like to make grown men cry - by all means enable this deathtrap.